In this paper, we introduce a simple, randomized dynamic data structure for storing multidimensional point sets, called a quadtreap. This data structure is a randomized, balanced variant of a quadtree data structure. In particular, it defines a hierarchical decomposition of space into cells, which are based on hyperrectangles of bounded aspect ratio, each of constant combinatorial complexity. It can be viewed as a multidimensional generalization of the treap data structure of Seidel and Aragon. When inserted, points are assigned random priorities, and the tree is restructured through rotations as if the points had been inserted in priority order. In any fixed dimension d, we show it is possible to store a set of n points in a quadtreap of space O(n). The height h of the tree is O(log n) with high probability. It supports point insertion in time O(h). It supports point deletion in worst-case time O(h2) and expected-case time O(h), averaged over the points of the tree. It can answer ε-approximate spherical range counting queries over groups and approximate nearest neighbor queries in time O(h + (1/ε)d-1).